The present invention relates to the elimination of scattered photons from transmission and emission images and, in particular, to the elimination of such photons as a function of pixel location.
The problem of Compton scattering is well-known in gamma cameras. This scattering results in photons appearing to have originated at incorrect locations. Some of these photons can be ignored on the basis of their significantly reduced energy, but others have energy levels comparable to the unscattered photons.
In the typical 50 to 500 keV operating region of gamma cameras, small scattering angles are much more probable than large angles. Therefore, scattered photons still have a high probability of passing through a collimator. The finite energy resolution of gamma cameras (8 to 10 percent) prevents discrimination of the slight energy shift caused by scattering. Because of varying angle, scattered photons detected in the camera point in the "wrong" direction and decrease image quality. Because of the detection of photons emitted not only from the region of interest inside the patient body but also photons scattered from other regions, scattered photons make accurate quantitative analysis impossible.
Traditional SPECT cameras are increasingly being employed for transmission scanning operations in conjunction with the standard emission study. A transmission scan uses an external radioactive source to quantify distribution of tissue densities. The information is used to correct for the nonuniform attenuation during emission study.
The photons of the transmission source are subject to the same Compton scattering process that occurs with emission photons. No current transmission scanning approach uses scatter elimination techniques to remove Compton-scattered events form the transmission images. The quantitative accuracy of the attenuation maps derived from these studies is therefore undefined.
Scatter elimination improves the resolution of an image, increases the contrast between "hot" (or radioactive high uptake) areas and "cold" ones in an emission study, allows measurement of attenuation coefficients for "narrow beam" geometry for a transmission study, and makes accurate quantitative analysis possible for both types of studies.
Because of the nature of emitted photons and the method of their detection, the energy of photons can be measured only with limited accuracy. Instead of a single spectral line in the energy domain, one has to deal with a photon spectrum or energy distribution (the number of events within a certain energy ranges). The spectrum is characterized by its shape and a few parameters, such as, the energy corresponding to a centroid of a spectrum, and spectrum full width at half of maximum (FWHM). It is a well known fact that the spectrum of "direct" unscattered photons is characterized by a Gaussian distribution.
It is important to keep in mind the Compton scattering is object dependent, because it depends on the distribution of the radioisotope inside the patient's body. Also, it is a three-dimensional phenomenon, since photons are emitted isotropically within the subject. In addition, it is not space uniform: it varies across the camera surface. This non-uniformity in the number of scattered photons detected by the camera results from its dependence on the subject, that is, the materials through which the photon should pass before detection by a camera. Therefore, the scatter content of an emission image will vary from patient to patient, from view to view at various angles around the same patient, and from area to area within the same view.
Various spatially independent methods have been proposed for reducing the scatter error, but these methods suffer greatly from the fact that the scatter error is typically very spatially dependent. The scatter error will spatially vary from patient to patient, from view to view at various angles around the same patient, and from area to area in the same view.
Various spatially dependent methods have also been proposed. These include dividing the photopeak region into two subwindows where the ratio of counts (or subwindow sizes adjusted to equal counts) is assumed to be a measure of scatter content. A fraction of events is removed from the lower subwindow on a pixel by pixel basis. This ignores the fact that many scattered photons will have been collected in the upper subwindow.
U.S. Pat. No. 5,371,672 estimates the number of scattered photons detected in each pixel from the number of events acquired within two narrow windows located on each side of the photopeak window. The contribution of the scattered events in the photopeak window is estimated as a trapezoidal area calculated to fit the counts in the narrow subwindows. The very small subwindows result in very noisy counting statistics and unreliable results on a pixel by pixel basis. In addition the assumed trapezoidal shape of the scatter function is not quantitatively accurate.
U.S. Pat. Nos. 4,839,808 and 5,081,581, incorporated herein by reference, require knowledge of a scatter-free spectrum and assumes the scatter spectrum can be represented by a third-order polynomial. For each pixel, the local spectrum is modelled as the sum of a polynomial approximating the scatter spectrum and a scatter-free spectrum multiplied by a constant. Each measured spectrum is fitted to this model to obtain polynomial coefficients and the constant. In practice, this method requires fine spectrum resolution and is very unstable, especially for pixels with low counts. The procedure for fitting spectra with two different functions assumes knowledge of these functions, for example, the order of the scatter spectrum polynomial. The method also requires knowledge of the camera's response with respect to photon energy.
U.S. Pat. No. 5,315,506 adds an energy regularization term which allows the use of separate parameters for each energy channel within the fitting window. In this method higher order polynomials can be used or non-polynomial functions. This method also suffers from requiring knowledge of the scatter-free response of the camera and is even more computationally demanding.
U.S. Pat. No. 5,293,195 also uses a spectrum fitting approach. For the scatter function, a calculation of the probability of how many scatter interactions a photon has undergone based on the Nishina-Klein equations is performed. Besides the requirement of knowing the scatter-free response of the camera, this method requires knowledge of the relevant physical model for the scattering spectra. Lack of this knowledge results in substantial inaccuracy.
The weakness of all of these methods are exacerbated in the case of combination transmission and emission imaging. This type of imaging is finding increased use in a quest to provide corrections for image inaccuracies caused by variations in the subject's composition and geometry. The traversing of the entire subject by transmission photons greatly increases the opportunity for scatter.